In an angular code encoder known from the USSR Author's Certificate No. 1474843, Int. class H03M 1/24, published in 1989, the reduction of a required shift of a signal readout unit is reached by the use of a row of light detecting windows, the number of the windows being determined by the digit capacity of the code used.
Although in this case the travel length of the preliminary moving of the readout unit is drastically reduced, it is not eliminated, which is explained by ambiguity of reading out the code of the readout unit position by light detecting windows of the analyzing mask.
The ambiguity of reading out the coordinate code can arise in case of a “critical” arrangement of the boundary of the code window with reference to the center of one of the analyzing windows, that is when it is not possible to reliably allocate values “0” or “1” to a signal of a respective light detector. Therefore, in order to eliminate the ambiguity in reading out the coordinate code, the analyzing mask (i.e. the readout unit) has to be moved additionally.
Hence, when determining the coordinate code after switching on the encoder, the above-mentioned shift of the readout unit becomes mandatory, which is inadmissible in some occasions.
Another position encoder is known from the U.S. Pat. No. 5,235,181, Int. class G01D 5/34, published in 1993, and chosen as the prototype for this invention of a code-to-position optoelectronic encoder.
This encoder contains a coordinate scale with a raster track and a code track, the first one being made as a chain of windows forming a regular raster and the second one being made as a code chain of windows, a readout unit comprising a lighter, a lens, projecting the illuminated track of the code windows onto a light-sensitive surface of a CCD (multi-element light detector), and a raster analyzing mask, which forms optic raster conjugation with the raster track of the coordinate scale and respective light detectors arranged behind it. The encoder also contains recording and analyzing units. The CCD detector of this encoder generates a video signal, which is a chain of unipolar pulses, the envelope of which is adequate to the distribution of luminance over its light-sensitive surface. Hence the video signal fully represents a current image of a section of the code track in the coordinates of the CCD array.
The analysis of the video signal, which corresponds to the coordinate code, makes it possible to determine the position of the readout unit, with the ambiguity of reading out the code information being obviated.